Nitrogen fixation in marine Oscillatoria appears to be associated with differentiated cells located in the center of the colony. These central cells exhibit reduced pigmentation relative to peripherally located cells and do not incorporate 14CO2 in photosynthesis. Central cells apparently do not produce O2 which would deactivate nitrogenase. When central cells are exposed to O2 via disruption of the colonies, N2 fixation (acetylene reduction) decreases sharply even though individual trichomes remain intact. Disruption of colonies in the absence of O2 does not cause reduced nitrogenase activity. In the sea, turbulence from wave action apparently separates trichomes allowing O2 to enter thus decreasing nitrogenase activity. These observations explain how Oscillatoria is able to fix N2 without heterocysts in an aerobic environment and why it blooms virtually always occur in calm seas.

From data in the literature on relative rates of copolymerization it has been possible to evaluate two constants, Q and e, characteristic of an individual monomer, which appear to account satisfactorily for its behavior in copolymerization. The constant Q describes the “general monomer reactivity” and is apparently related to possibilities for stabilization in a radical adduct. The constant e takes account of polar factors influencing copolymerization.

Ah,stlnct Measurements of Oscillutoria spp. standing crop, primary production, and N:! fixation were made at 46 stations on three cruises to the Caribbean and wcstcrn Sargasso Seas. Oscillatoria is a major component of the Caribbean Sea plankton, comprising 60% of the total chlorophyll CL in the upper 50 m and about 20% of the primary production. Through N, fixation it contributes an avcragc of 1.3 mg N m-" d-' to the Caribbean Sea, and this may bc a major input of nitrogen to this arca. Oscillatok appears to bc of relatively little importance in the western Sargasso Sea. Nitrogen fixation by the planktonic blue-green algae Oscillatoria (Trichodesmium) spp. is potentially an important source of nitrogen for organisms inhabiting the open sea (Dugdalc et al. 1961, 1964; Goering et al. 1966). IIowever, Mague ct al. (1974) and Carpenter and McCarthy (1975) calculate for the central Pacific and western Sargasso Sea, respectively, that NZ fixation rcprescnts but a minor source of new combined N. Refore any firm conclusions as to the nonimportance of OsciZZatoriu in the N cycle of the sea are reached, it is imperative that further measurements of its density and N2 fixing activity bc made in these and other arcas. We present here data on the standing crop, productivity, and NZ fixation rates of Oscilkltorin in both the western Sargasso and Caribbean Seas. We thank the officers and crew of the RV Knorr and RV Atlantis II for their help, advice, and interest in this project. Analysis of j5N was carried out by J. J. McCarthy. Methods Samples were collected on three cruises: RV Knorr cruise 33 (K33) to the western 1 This research is support& by National Sciencc IJoundation grant GA 37993. Contribution

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